Yes, I reckon it might get more usage there to. I guess the question that needs to be asked here is..

Does the custom mode stuff only exist in live lobbies because it wasn't possible to implement it into async lobbies? or, was it a conscious design decision to exclude it from async (perhaps to try to promote live only?).

I haven't checked out how it all works yet so perhaps there's an obvious reason why it's specific to live games only. If however it's a design decision, all that does is exclude the long-term player-base in an attempt to appeal to a short-term player-base..

..similar to what happened with 1v1 chaos, re-focusing the format to be about quick games on novelty sized maps in an attempt to promote live action just sidelined the players who specialized in 1v1 only.

As for custom modes as a concept, there may have been a missed opportunity here to consult the player-base over what would have worked for this. There are many unofficial Chaos variants (with simple custom rules) that players are aware of which could have made ideal custom modes.

Has this been implemented? Or how hard might it be? Even without god rewards, just being able to set objectives and taboos would be an immense improvement as async games seem to be all i can play lately.

The taboo options in classic mode have so much potential to be wasted on players who aren't ever into playing custom games. (setting up super specialized wizard scenarios would be fun as all hell)

Being able to shoe horn a totem in place as a wizard would be great as well. I remember playing around with custom rules were players would have to stand still (like a totem) and rely on their spells. Even if they were just plopped in as an objective category like:

and simply spawn a totem with troops, this could be a fun way to have a more base assault/defense dynamic.